Intra-articular tissues, such as the anterior cruciate ligament (ACL), do not heal after rupture. In addition, the meniscus and the articular cartilage in human joints also often fail to heal after an injury. Tissues found outside of joints heal by forming a fibrin clot, which connects the ruptured tissue ends and is subsequently remodeled to form scar, which heals the tissue. Inside a synovial joint, a fibrin clot either fails to form or is quickly lysed after injury to the knee, thus preventing joint arthrosis and stiffness after minor injury. Joints contain synovial fluid which, as part of normal joint activity, naturally prevent clot formation in joints. This fibrinolytic process results in premature loss of the fibrin clot scaffold and disruption of the healing process for tissues within the joint or within intra-articular tissues.
Enhancing healing of ligaments using growth factors has been an area of great interest and research. While the majority of studies have focused on the use of a single growth factor to stimulate healing, the natural wound healing process is an orchestration of multiple growth factors released by platelets and other cells over time. To try to reproduce this in the in vitro and in vivo environment, prior investigators have looked at sustained release carriers and viral vectors for release of these cytokines over days or weeks, as well as examining applications of multiple growth factors. These studies have shown some additive effects of applied combinations of growth factors on the wound healing of ligaments; however, even with advanced application techniques, the combinations of growth factors, timing of release and concentration of release make optimization of these systems difficult.
An alternative method recently used to stimulate healing of the anterior cruciate ligament is the application of activated platelet-rich plasma (PRP). PRP is a combination of the extracellular matrix proteins normally found in plasma (including fibrinogen and fibronectin) and platelets. When platelets are activated by the exposed collagen of a ligament injury, they begin to aggregate and release multiple growth factors including platelet-derived growth factor (PDGFαα, PDGF αβ, PDGF ββ), transforming growth factors-β (TGFβ1, TGFβ2), vascular endothelial growth factor, basic fibroblast growth factor (FGF2), IGF-1 and epithelial growth factor. Growth factor release typically occurs immediately upon platelet activation and is sustained at much lower levels for the life-span of the platelet—up to 5-7 days.
PRP can be used to increase local concentrations of active PDGF-αβ and TGF-β1 by over 300% when platelets are concentrated in the plasma to a similar degree. This degree of platelet concentration can be accomplished by several available systems. As seen in vivo, these levels of cytokines released locally by these platelet concentrates can result in increased fibroblast DNA synthesis and up-regulation of type I collagen production and changes in collagen organization, and indeed the use of far lower concentrations (10 ng/ml TGF-β1 and 20 ng/ml PDGF-αβ can influence fibroblast proliferation, fibroblast chemotaxis, collagen production and collagen organization. The use of PRP over purified growth factor concentrates provides the added benefit of additional ECM proteins which also stimulate cellular adhesion and collagen synthesis, particularly in the presence of collagen fibrils.